Older vegetarians matched meat eaters' strength gains from lifting while adding less muscle

Vegetarians tend to eat less protein than meat eaters ^[12]. A trial that let intake float would mostly compare protein doses. The Amsterdam trial set a protein floor. Diets were randomized ^[2], and every participant was coached toward at least 1.2 grams of protein per kilogram of body weight a day ^[4].

For strength and function, the protein source did not appear to matter. Both groups improved, with no significant difference between them ^[6]. Muscle mass was another matter. Arm and leg muscle grew by 0.66 kg in the omnivores and 0.38 kg in the vegetarians ^[8]. That is a gap of 0.28 kg ^[13], and the omnivore gain is about 1.74 times the vegetarian one ^[14]. ScienceAlert describes the difference as about 70 percent more ^[8].

ScienceAlert's account leaves two explanations open: the vegetarians ate less protein, or animal protein does more for muscle ^[15]. Both groups met their targets on average ^[5]. The article does not report each group's actual intake, so the vegetarians could have cleared the floor and still eaten less than the omnivores.

The most informative result sits inside the vegetarian arm. Vegetarians who ate more protein tended to gain more muscle, while meat eaters who ate more did not ^[16]. "The omnivorous group likely reached a saturating protein threshold where additional intake yielded no further anabolic benefit," the authors wrote ^[17]. It is a plausible hypothesis, and the trial was not built to test it. Participants were coached to a minimum ^[4]. How far each person went above it was not randomized, so this pattern is a correlation inside a randomized trial.

In my view the trial supports a conditional claim. At 1.2 g/kg, older vegetarians got the same strength and function gains from 12 weeks of training as meat eaters ^[6]. The trial cannot say whether that intake is enough for the full muscle gain, or whether more plant protein would close the 0.28 kg gap ^[13]. The researchers' own conclusion is also conditional. "Importantly, our results also show that a predominantly plant-based diet can still support meaningful improvements in muscle mass, strength, and physical function when protein intake is sufficiently high and combined with resistance training," they wrote ^[10].

They added a caveat aimed at the people least likely to hit a target: "For people with low appetite or low energy intake, extra focus on meeting protein targets and ensuring protein quality is likely needed to maintain muscle mass, which is particularly important for older adults." ^[11]

The thing this doesn't tell you is what happens to a 0.28 kg gap measured after three months ^[13][2] if the training continues for years.